The most popular theory
of why we find jokes funny revolves around the concept of ‘incongruity’.

The idea is that we laugh at things that surprise us because they
seem out of place. It’s funny when clowns wear outrageously
large shoes, people have especially big noses or politicians tell
the truth.

In the same way, many jokes are funny because they involve ideas
that run against our expectations. A bear walks into a bar. Animals
talk. And so on.
But there is more to this theory than such simple forms of incongruity.
In many jokes, there is an apparent incongruity between the set-up
and the punch line.

Take the following joke:
Two fish in a tank.
One turns to the other and says: “Do you know how to drive
this?”

The set-up line leads us to think about two fish in a fish tank.
But the punch line surprises us – why should the fish be able
to drive a fish tank? Then, a split second later, we suddenly realise
that the word ‘tank’ has two meanings, and that the
fish are actually in an army tank.

Scientists refer to this as the ‘incongruity-resolution’
theory. We resolve the incongruity caused by the punch line, and
the accompanying feeling of sudden surprise makes us laugh.

The following LaughLab jokes illustrate different kinds of incongruity:

Did you hear about the man who drowned in a bowl of muesli?
He was pulled under by a strong currant!

I said to the Gym instructor: “Can you teach me to do the
splits?” He said: “How flexible are you?” I said:
“I can't make Tuesdays.”

Two owls are playing in the final of the Owl Pool Championship.
It comes down to the last frame. One of the owls is just about to
play his shot, when his wing accidentally touches a ball. “That's
two hits,” says the other owl. “Two hits to who?”
says the first.

Did you hear about the ice-cream man, he was found dead in his ice-cream
van, covered in chocolate sauce and hundreds-and-thousands? The
police said that he had topped himself.

Two aerials met on a roof, fell in love and got married.
The ceremony was rubbish but the reception was brilliant.